1. Field of the Invention
The present invention relates to preactivated unsupported catalyst compositions and to methods of using them whereby the compositions have a concentration of preactivated catalyst within the range of from about 0.04 to about 0.1 mmol of preactivated catalyst per liter of solution when using aliphatic or alicyclic hydrocarbon solvents, and a concentration of less than about 0.80 mmol/liter when using aromatic or halogen-substituted solvents. In the method, an unsupported catalyst precursor first is contacted with an activator, or co-catalyst, in a suitable reaction medium, and then the resulting mixture is contacted with additional solvent to form a preactivated unsupported olefin polymerization catalyst composition that can be fed to a gas phase polymerization reactor without plugging the catalyst injection nozzle. Combining the unsupported catalyst precursor, the co-catalyst and then adding additional solvent to provide a composition having a concentration within the range of from about 0.04 to about 0.1 mmol/liter (using aliphatic or alicyclic hydrocarbon solvents) or less than about 0.80 mmol/liter (using aromatic or halogenated hydrocarbon solvents) prevents tube plugging, and provides a catalyst material that has high activity, avoids forming significant amounts of polymer agglomerates, and avoids reactor fouling.
2. Description of Related Art
Gas phase polymerization of olefin monomers to produce polyolefins is well known in the art. Various polyolefins can be produced including homopolymers, copolymers and terploymers of xcex1-olefins and optionally including dienes, aromatic compounds with vinyl unsaturation and/or carbon monoxide. A catalyst typically is required to initiate polymerization of one or more of the xcex1-olefin monomers, and the optional dienes, etc. Typical catalysts include, but are not limited to, coordinated anionic catalysts, cationic catalysts, free-radical catalysts, anionic catalysts and the like. As described more fully, inter alia, in U.S. Pat. Nos. 3,779,712, 3,876,602 and 3,023,203, these known catalysts are introduced to the reaction zone as solid particles whereby the active catalyst material is supported on an inert support typically made of alumina, silica and the like. It was generally known in the art that delivering conventional catalysts to a gas phase reactor that were unsupported would result in numerous problems in catalyst delivery, as well as undesirable polymer properties.
Recent developments in the industry, however, have led to the discovery of a class of unsupported catalysts, some of which are typically referred to as metallocenes, or single site catalysts. Delivery of liquid, unsupported catalysts to a gas phase reactor was first described in Brady et al., U.S. Pat. No. 5,317,036, the disclosure of which is incorporated herein by reference in its entirety. Brady recognized disadvantages of supported catalysts including, inter alia, the presence of ash, or residual support material in the polymer which increases the impurity level of the polymer, and a deleterious effect on catalyst activity because not all of the available surface area of the catalyst comes into contact with the reactants. Brady further described a number of advantages attributable to delivering a catalyst to the gas phase reactor in liquid form.
These advantages included a cost savings since there were no costs associated with providing the support material, and processing the support so as to impregnate the active catalyst thereon. In addition, a high catalyst surface area to volume ratio was achieved thereby resulting in improved catalytic activity. Moreover, it was more efficient since the catalytic solid no longer needed to be separated and processed (filtered, washed, dried, etc.), and then handled and transported.
Despite these advantages, the solid catalytic material still needed to be dissolved in a suitable solvent and delivered to the gas phase reactor in the solvent. Many, if not all, of the single site metallocene catalysts which may polymerize olefins, and especially propylene isotactically, such as metallocene dichlorides, are difficult to use because they are insoluble in hydrocarbon solvents such as alkanes. Other unsupported catalysts that may polymerize olefins also are not readily soluble in hydrocarbon solvents, or require significant amounts of hydrocarbon to dissolve the unsupported catalysts. Solvents such as toluene and methylene chloride, although capable of solvating such catalysts, are undesirable because they are toxic in nature and leave undesirable residues. Even in these types of solvents, however, solubilities still can be very low, typically less than 21 mmol/l in concentration at room temperature. In addition, feeding unsupported catalysts to a gas phase reactor using large quantities of solvents (hydrocarbon or otherwise) can cause reactor fouling to occur, as described, for example, in U.S. Pat. No. 5,240,894.
In addition, when a liquid catalyst is employed in gas phase polymerization, several phenomena can occur. First, the soluble or liquid catalyst tends to deposit on the resin or polymer forming the fluidized bed which in turn leads to accelerated polymerization on the surface of the particles of the bed. As the coated resin particles increase in size, they are exposed to a higher fraction of catalyst solution or spray because of their increased cross-sectional dimensions. If too much catalyst is deposited on the polymer particles, they can grow so large that they cannot be fluidized thereby causing reactor shut down.
Second, using liquid catalyst under conditions of high catalyst activity, e.g., a liquid metallocene catalyst, the initial polymerization rate is often so high that the newly formed polymer or resin particles can soften or melt, adhering to larger particles in the fluidized bed. This needs to be avoided or minimized to avert reactor shutdown.
On the other hand, if the polymer particle size is too small, entrainment can occur resulting in fouling of the recycle line, compressor, and cooler and increased static electricity can occur leading to sheeting,xe2x80x94and ultimately, reactor shut down.
It also was generally thought in the art that introduction of liquid catalyst to a gas phase polymerization would result in small particle sizes, cause undesirable swelling of the polymer or, at the very least, cause aggregation and agglomeration in the particle bed. This agglomeration would undesirably not fluidize well. Agglomerates would plug the product discharge valve, coat the walls of the reactor and form sheets, disrupt the flow of solids and gas in the bed, and generate large chunks that may extend throughout the reactor. Large agglomerates also can form at the point of introduction of the liquid catalyst and plug the catalyst injection nozzle or tube. This may be in part due to the excess amount of hydrocarbon needed to dissolve the unsupported catalysts. Moreover, carry over of excess liquid occurs, causing an undesirable catalyst coating of the walls of the heat exchanger and other downstream equipment with polymer.
It is known to contact single site catalysts that are soluble in hydrocarbons with a coactivating cocatalyst solution prior to administering the catalyst solution to the gas phase reactor, as described, inter alia, U.S. patent application Ser. Nos. 08/781,196 filed Jan. 10, 1997, now U.S. Pat. No. 5,912,202 and 08/782,499, filed Jan. 10, 1997, now abandoned, the disclosures of which are incorporated by reference herein in their entirety. The amount of hydrocarbon needed to dissolve the catalyst precursor, however, can be so high to result in an ultimate catalyst solution whose concentration is low enough to cause coating of existing resin particles in the gas phase reactor when the catalyst solution is introduced. This coating phenomenon forms undesirable agglomerates and xe2x80x9cchunksxe2x80x9d of polymer resin material. This problem is exacerbated when the unsupported catalyst is insoluble in hydrocarbons, or only slightly soluble in hydrocarbon solvent.
Preactivating an unsupported catalyst precursor with a co-catalyst may be sufficient to enhance the solubility of the unsupported catalyst, and serves to reduce the need to use toxic solvents, or high quantities of solvent. Plugging of the catalyst feed tube still may occur, however, if the concentration of the unsupported catalyst is too high. In addition, if the concentration of the unsupported catalyst is too low, too much liquid may be introduced into the reactor causing coating of the resin particles, as described above.
Thus, there exists a need to develop a mechanism by which unsupported catalysts can effectively be delivered to a gas phase polymerization reactor without causing the catalyst feed tube to plug. There also exists a need to develop methods of delivering unsupported catalysts to a gas phase reactor without causing polymer agglomeration, and without causing reactor fouling. It is therefore an object of the invention to provide an unsupported catalyst system and method of polymerization that does suffer from the aforementioned problems, and that satisfies the needs discussed above.
In accordance with these and other objects of the present invention, there is provided a preactivated unsupported olefin polymerization catalyst composition comprising an unsupported olefin polymerization catalyst precursor, a co-activator or co-catalyst and a solvent, whereby the concentration of the preactivated unsupported catalyst is within the range of from about 0.04 to about 0.1 mmol of catalyst per liter of solution when using an aliphatic or alicyclic hydrocarbon solvent, and the concentration is less than about 0.80 mmol/liter when using an aromatic or halogen-substituted solvent. In accordance with an additional object of the present invention, there is provided a method of making a polymer in a gas phase polymerization reactor comprising contacting, in the gas phase, an olefin monomer with the preactivated unsupported olefin polymerization catalyst composition in liquid form. These and other objects of the invention will be readily apparent to those skilled in the art upon review of the detailed description that follows.